1. Field of the Invention
The present invention relates to a process for producing aluminum fluoride particles having a smooth surface and excellent fluidity. More specifically, it relates to a wet process for producing aluminum fluoride anhydride particles having excellent fluidity preferably with a repose angle of 30.degree. or less efficiently. In the following description, "%" means "% by weight" unless otherwise stated.
2. Description of the Related Art
Aluminum fluoride anhydride is widely used in glazes for ceramic wares, coatings for welding rods, fusing agents for aluminum electrolytic refining and the like. It is also used as a synthesizing catalyst for organic fluorides.
To produce this aluminum fluoride anhydride, there are known a dry process and a wet process. The dry process comprises fluorinating aluminum hydroxide with a hydrogen fluoride gas and the wet process comprises adding aluminum hydroxide to hydrofluoric acid to produce a super-saturated solution of aluminum fluoride and crystallizing aluminum fluoride from this solution.
Since the wet process can use fluosilicic acid (H.sub.2 SiF.sub.6) which is more inexpensive than hydrofluoric acid, it is industrially advantageous and further has such an advantage that a product having relatively higher purity than that of the dry process can be obtained. However, aluminum fluoride produced by the conventional wet process is inferior in fluidity because the surface of an aluminum fluoride particle is covered with needle-like or column-like crystals. For example, when aluminum fluoride particles are used as a flux for aluminum electrolytic refining, they are liable to clog a transport pipe or tank. Therefore, the dry process is now used in most cases.
As an example of the conventional wet process, U.S. Pat. No.3,057,681 discloses a process for producing aluminum fluoride particles which comprises a first step of producing a super-saturated solution of aluminum fluoride through a reaction between aluminum hydroxide and fluosilicic acid and a second step of adding aluminum fluoride seed crystals after silica is removed from this super-saturated solution to crystallize aluminum fluoride on the surface of the seed crystal so as to grow a particle.
In this process, although the difference of filterability of silica is studied by changing the reaction amounts (molar ratio) of aluminum hydroxide and fluosilicic acid in the first step, the influence of fluosilicic acid is not taken into account in the crystallization reaction of the second step. Further, a description of general reaction conditions such as solution concentration, liquid temperature, and the particle diameter and amount of the seed crystals added is given but the fluidity of the obtained aluminum fluoride particles is not studied at all. Therefore, as the repose angle of aluminum fluoride anhydride obtained through this crystallization reaction is about 40.degree., its fluidity is low.
Austrian Patent No.217,008 teaches a wet process for producing aluminum fluoride through a reaction between fluosilicic acid. In this process a 0.5 to 5 wt % excess of fluosilicic acid is reacted with aluminum hydroxide (equivalent to a reaction molar ratio of Al(OH).sub.3 /H.sub.2 SiF.sub.6 of about 1.90 to 1.99) to improve the production efficiency. This is aimed to increase the crystallization speed of aluminum fluoride and not to improve the fluidity of the obtained aluminum fluoride. In fact, aluminum fluoride particles obtained by this process have low fluidity with a repose angle of 40.degree. like those produced by other wet processes.